Diffraction grating couplers used as optical multiplexing or demultiplexing devices take light from the input fiber or fibers, respectively, and couple it back into output fibers or fiber, respectively. These couplers utilize a diffraction grating, that is, an angularly dispersive device, that diffracts away incident collimated light at an angle dependent upon the incidence angle and the wavelength of the incident light. In this way, light can be separated by wavelength and coupled as desired.
There are several types of diffraction grating couplers, one common type using a concave diffraction grating and another common type using a radially graded refractive index (hereafter GRIN) lens with a plane diffraction grating. The concave diffraction grating type device has the advantage of not requiring any light collimating and/or refocusing optics. Its disadvantages are that extremely tight control must be exercised in forming the spherical concave surface and also in forming the grating configuration. Compounding the latter requirement is the fact that the ruling tool used to form the grating must swing through an arc as it traverses the spherical surface. In addition, concave grating type devices have a low diffraction efficiency and can suffer from image astigmatism. The disadvantage of the GRIN lens type device is that it includes an additional optical device to collimate and focus the light.